1. Field of the Invention
The present invention relates to a surface light source device of the type having with a light guide plate provided with a primary light source disposed on one side thereof and one surface acting as an emission surface (hereinafter, referred to as "surface light source device of side light type"), and more particularly to improvements in the surface light source device of side light type having a light guide plate with a back surface on which a distribution pattern of light diffusible elements is provided. The surface light source device of side light type in accordance with the present invention is specifically adaptable for use as a back-light source for liquid crystal display.
2. Related Art
A description will first be given on the typical structure and operation of a surface light source device of side light type. FIG. 1 is a main-part cross-sectional diagram showing the basic structure of a prior known surface light source device of side light type along with an arrangement for brightness measurement as described later. In this figure, the numeral 2 designates a light guide plate made of a transparent plastic material. On the side of light guide plate 2, a rod-shaped light source element 1 is disposed as the primary light source along the side edge surface (incidence surface) 21 of the light guide plate 2. Typically, a cold cathode tube such as a fluorescent lamp is employed for the light source element 1. The light source element 1 will be simply called the "lamp" hereinafter.
One surface of the light guide plate 2 acts as an emission surface 22. In order to facilitate retirement of the uniformity of brightness for emission surface 22, the cross-sectional shape of light guide plate 2 is designed in most cases so as to resemble a wedge with its lamp side being increased in thickness. One or a plurality of sheet-like elements 3 (referred to as "additional elements" hereinafter) are additionally disposed on the emission surface 22. The additional element(s) 3 may include a light diffusion sheet, prism sheet, prism protective sheet, interference-pattern elimination sheet, and others.
A reflector 4 made of a light reflection sheet is disposed along the opposite surface 23 (hereinafter, referred to as "back surface" while being called "reflection surface" in some cases) to the emission surface 22. Silver thin-film or foil, aluminum foil, white sheet or the like may be used for reflector 4. Although illustration is omitted, a reflector is arranged on the rear-surface side of lamp 1 for enhancement of the incidence efficiency of light to the light guide plate 2.
Most light rays radiated from the lamp 1 are introduced directly or indirectly from the light guide plate 21 into the inside of the light guide plate 2. The light introduced into light guide plate 2 is optically guided to travel toward a terminal surface 24 while it is repeatedly reflected at the front and back surfaces 22, 23 of the light guide plate 2. During such processes, illumination light will be gradually taken out of emission surface 22, finally providing illumination light through the action (light diffusing action, etc.) of the additional element 3. This illumination light will be utilized as back-light illumination of a liquid crystal display, for example.
In order to attain uniform brightness of the emission surface 31 as formed on the outside surface of additional element 3 and thereby to direct the light introduced inside light guide plate 2 to the side of emission surface 22 positively, a number of elements having light diffusing ability or "diffuseability" are disposed on the side of back surface 23 of the light guide plate 2 with a certain distribution pattern established. This distribution pattern is also called the "dot pattern" due to the fact that the pattern consists of multiple dot-shaped elements arranged according to certain rules.
FIG. 2 is a diagram for explanation of the basic form of such distribution pattern of the light diffusible elements, wherein the light guide plate 2 is depicted in plan view as looked at from the side of its back surface 23. In this drawing, respective light diffusible elements 50 are illustrated with a number of marks ".quadrature." in various sizes. Each light diffusible element 50 is formed as a print layer of light diffusible inks, a satin finish surface (fine unevenness surface), or the like. The sizes of light diffusible elements 50 are so designed as to exhibit the minimum value Dmin at the nearest array to the lamp 1 having an electrode section 11, whereas it takes the maximum value Dmax at the farthest array from lamp 1.
The multiple light diffusible elements 50 in this example are provided in a matrix shape with pitch d defined in common for the longitudinal and transverse directions thereof. Regarding the distribution pattern of light diffusible elements 50 and the shape of the individual light diffusible element 50, a lot of modifications have been proposed. In any case, the concept of increasing the covering rate with distance from lamp 1 is employed. Here, the term "covering rate" refers to the "the area occupation ratio of the region having the light diffuseability." This will be called "pattern covering rate" hereinafter. The above concept is by taking reasonable viewing from general demands for uniform brightness over the entire area of the emission surface 22.
A problem with the case where such light diffusible elements 50 are provided on the back surface 23 of light guide plate 2 is what is called the "pattern see-through". This pattern see-through is a phenomenon that the pattern of light diffusible elements 50 formed on the back surface 23 of light guide plate 2 is observed through the side of emission surface 22; it tends to be striking as the light guide plate 2 gets smaller in thickness and as the light diffusible elements 50 increase in pattern pitch d.
When the surface light source device with visually recognizable pattern see-through is used as a back light for liquid crystal display, this will obviously degrade display quality. In accordance with a prior art, in order to eliminate the pattern see-through, a light diffusion sheet having strong light diffuseability is used as the additional element 3 arranged on the emission surface 22. According to another prior art, the pattern see-through was prevented by employing a plurality of light diffusion sheets overlapped each other.
Furthermore, it has been attempted to avoid see-through by employing two or more prism sheets capable of increasing the illumination light toward the front direction.
However, the method of erasing pattern see-through by reinforcement of light diffusion action suffers from a difficulty that the brightness as the surface light source device decreases simultaneously. Moreover, once the output light from emission surface 22 looses directivity due to such strong light diffusion action, it becomes difficult for the prism sheet to offer its inherent actions sufficiently.
FIG. 3 is a graph showing this situation as a practical example. Layout of brightness measurement deriving the results of this graph is schematically coindicated in FIG. 1. Specifically, a brightness meter M is located in a rotation-scannable way under the condition that it looks at the center point P of the outermost additional element 3 or emission surface 22 (no additional element 3) from a position as spaced apart from it by a predefined distance (320 mm). While scanning is done in a range of the angle .theta. (transverse axis of graph),with respect to a perpendicular N upstanding at the center point P to cover from about -80 degrees to +80 degrees, brightness values (ordinate axis of graph) were plotted in "nt" (candela/m.sup.2).
Curves A, B, C represent measurement results as to the cases of "light guide plate alone (without use of additional elements)", "light guide plate+conventionally widely commercially available ordinary light diffusion sheet" and "light guide plate+conventionally widely commercially available ordinary light diffusion sheet+prism sheet (by Mitsubishi Rayon: DIAART H159 trade name! with the prism vertical angle of 90 degrees) disposed outside it", respectively.
The size of light guide plate 2 is L1=166 mm, L2=219 mm; the thickness on the side of incidence surface 21 is 3 mm; the thickness on the side of terminal surface 24 was 1 mm. Then, the pitch between adjacent light diffusible elements 50 is d=1.0 mm; the minimum size Dmin=0.34 mm; the maximum Dmax=0.8 mm. The prism sheet was arranged so that the prism arrays (groove arrays) are oriented parallel with the lamp, and that the prism surface faces outward. From the graph, the following can be seen.
1. Curve A: the output light from the emission surface 22 of light guide plate 2 is mainly emitted in the forward direction while slightly rising or upstanding from emission surface 22, whereas almost no light rays are output around the front direction (250 nt or less). PA1 2. Curve B; the conventional light diffusion sheet acting on the output light from emission surface 22 of light guide plate 2, the light propagation direction is forced to diffuse in a wide range causing resultant directivity of illumination light to become weak extremely. It should be noted that the light diffusion action provides a remarkably improved brightness (by 400 nt or more) in the front direction (around directions of .theta.=0 degrees) as compared with the case of using light guide plate 2 alone. The brightness of 500 nt or greater was obtained in a wide angular range of from .theta.=20 degrees to 70 degrees. PA1 3. Curve C; the prism sheet further disposed outside the conventional light diffusion sheet, most part of light is collected in the front direction resulting in that the brightness as looked at from around .theta.=0 degrees increases significantly (up to about 700 nt to 750 nt). However, the brightness distribution remains slightly deviated in the forward direction while the brightness on the .theta.&lt;0 side is somewhat lowered. PA1 1. In the use of the light guide plate alone (curve A), pattern see-through was clearly observed on the emission surface 22. In the case of observation through a liquid crystal panel, pattern see-through was also recognized. PA1 2. In the case of the light guide plate+light diffusion sheet (curve B), pattern see-through was recognized a little on the emission surface 31. However, when observing through the liquid crystal panel, no pattern see-through was recognized. PA1 3. In the case of light guide plate+light diffusion sheet+prism sheet (curve C), pattern see-through was recognized a little on emission surface 31; however, when observing through the liquid crystal panel, no pattern see-through was recognized.
On the other hand, the evaluation as to the pattern see-through was as follows.
Note that although not described in FIG. 3, when another prism sheet (two in total) is disposed outside the arrangement from which curve C was obtained, the brightness as looked at from the front direction (around .theta.=0 degrees) was further increased obtaining the value of 1,050 nt or around it. Additionally, regarding the pattern see-through, a little further improvement was recognized as compared with the case of curve C.